1. Field of the Invention
This invention relates generally to channel selecting apparatus for a television receiver, and more particularly is directed to a channel selecting apparatus for a television receiver having a so-called electronic tuner employing, as its tuning element, a varactor or voltage-controlled, variable reactance device, such as, a variable capacitance diode.
2. Description of the Prior Art
Most existing television receivers employ electro-mechanical tuners in which a channel-selecting knob is manually rotatable through various positions established by a detent mechanism and which respectively correspond to the 12 vhf channels. At each of such positions of the knob, suitable switch contacts are engaged to activate a respective circuit by which the receiving frequency for the designated channel is obtained. Such electro-mechanical tuners are susceptible to failure because of loose or dirty switch contacts and defects in the detent mechanism. Furthermore, electro-mechanical tuners of the described type become extremely complex and even more unreliable when designed to tune the uhf channels in addition to the vhf channels.
In order to overcome the above problems of electro-mechanical tuners, so-called electronic tuners have been proposed for television receivers. In these proposed electronic tuners, a varactor, that is an analog voltage-controlled, variable reactance device, such as, a variable capacitance diode, is employed as the tuning element, and the control voltage therefor is usually obtained either by means of a potentiometer array or a phase-locked loop arrangement. In the case of the potentiometer array, a number of potentiometers are connected between a stable voltage supply and ground, and each potentiometer is adjusted to provide a voltage which, when applied to the varactor by way of a respective solid state switch, will tune the receiver to a respective one of the local channels. Further, manually operable switches are provided to control the solid state switches associated with the several potentiometers. In the foregoing arrangement, it is difficult and costly to obtain stable potentiometers, and rather elaborate mechanical assemblies are required to provide for the adjustment of the several potentiometers so as to correspond to respective local channels.
In the existing electronic tuners employing a varactor with a phase-locked loop, the variable frequency output of the varactor controlled tuner is applied through an amplifier to a prescaler which divides such output frequency by a fixed number, and the resulting divided frequency is then further divided, in a variable counter or divider, by a number that is determined by a preset logic controlled by manually operable channel-selector switches. The output of the variable counter or divider is then compared, in a phase comparator, with a stable reference frequency obtained, for example, from a crystal-controlled oscillator, with the resulting error signal being used to control or vary the control voltage for the varactor. The foregoing phase-locked loop arrangement is disadvantageous in that the amplifier used to raise the low level of the output of the tuner for driving the digital prescaler may be costly, and further in that such prescaler has to operate at an undesirably high input frequency.
The present inventors have developed a channel selecting apparatus for a television receiver having a tuner with a voltage-controlled variable reactance device as its tuning element and which avoids the above described problems, for example, as disclosed in U.S. patent application Ser. No. 716,702, filed Aug. 23, 1976, and corresponding to Japanese Patent Application No. 103,668/75, filed Aug. 27, 1975. In such channel selecting apparatus, digital or binary codes representing or identifying respective channels are stored at respective addresses in a memory, and a digital-to-analog converter provides an analog control voltage for the variable reactance device in correspondence to each digital code selectively read out of the memory in a channel selecting mode of the apparatus. Further, in a programming mode of the apparatus, the changing digitally coded counts of a counter which is counting sweep pulses are applied to the digital-to-analog converter for similarly controlling the variable reactance device and selected counts of the counter, for example, those which result in the appearance on the receiver screen of pictures or test patterns broadcast by selected television stations or channels, are written at selected addresses in the memory as the channel identifying codes. Although the foregoing channel selecting apparatus is provided with a sweep pulse generating circuit which is manually controllable to provide either coarse or fine sweep pulses to be counted in the programming mode for establishing the channel identifying codes to be written or stored at selected addresses of the memory, the selection of each channel identifying code to be stored is made by visually observing the picture or image appearing on the screen of the television receiver and, therefore, the stored code may not be the optimum code for obtaining precise tuning to the respective channel.
In order to overcome the last mentioned problem, the present inventors, with another, have provided a channel selecting apparatus, as aforesaid, with an automatic fine tuning operation, for example, as disclosed in U.S. patent application Ser. No. 716,655, filed Aug. 23, 1976, and corresponding to Japanese Patent Applications No. 104,445/75, filed Aug. 28, 1975, and No. 104,556/75, filed Aug. 29, 1975. In such channel selecting apparatus, there are provided means for detecting deviation of the receiving frequency, as established in response to the analog control signal, from the correct receiving frequency for the channel identified by the digital code then being read out of the memory, and means responsive to a detected deviation of the established receiving frequency from the correct receivimg frequency for modifying the digital code then being received by the digital-to-anlog converter in the sense for removing said deviation, whereby to effect an automatic fine tuning operation. Further, the channel selecting apparatus is preferably provided with means operative, when the detected deviation of the established receiving frequency from the correct receiving frequency exceeds a predetermined amount, to store the resulting modified digital code in the memory in place of the read out digital code resulting in such deviation with a view to ensuring that the receiving frequency established at any time in response to a channel identifying code read out of the memory will remain within the pull-in range of the automatic fine tuning operation. Thus, the described channel selecting apparatus seeks to reliably achieve the automatic fine tuning operation even though the relation of the control voltage for the variable reactance device to the resulting receiving frequency may vary with temperature or the time of year. However, the automatic fine tuning in respect to each channel identified by a digital code stored in the memory is reliably achieved only if each such channel is regularly selected. In other words, if one or more of the channels programmed in the memory is infrequently selected, the change in the relation of the control voltage for the variable reactance device to the resulting receiving frequency may be so large during the extended time period between successive selections of such channel as to cause the receiving frequency established by the respective channel identifying code read out of the memory to be outside the pull-in range of the automatic fine tuning operation.